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Astigmatism

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Introduction

Astigmatism is one type of eye power. Eye Power can be spherical or cylindrical. The cylindrical type of eye power is also known as astigmatism. Some have only one type, and some have both spherical and astigmatism in their glasses. Corrective lenses overcome it in the glasses, and without glasses, one may get eye strain or have blurry vision.

Pronunciation

While in the clinic, many patients are not sure how to pronounce astigmatism. So here goes

Physics of a cylindrical Number

Emmetropia

Emmetropia
This image shows how rays of light bend when they enter the eye.  Rays from 360 all meet at one single point and we see clearly

Myopia

Myopia
Rays of light bend more and meet at a single point but not on the retina.  They meet in front of the retina leading to blurred vision.  This is myopia and is corrected by a minus powered or concave lens in glasses/contact lenses.

Hyperopia

Hyperopia
Rays of light bend less and meet at a point but behind the retina.  This is Hyperopia. This leads to blurry vision and is corrected by plus powered lenses or convex lenses.

Simple Myopic Astigmatism

simple myopic astigmatism
Rays of light from one plane meet on the retina but rays of light from another plane 90 degrees away from the first meet in front of the retina.  This is known as simple myoipc astigmatism.  Here the patient will have to wear glasses to improve vision.  The glasses will only have a cylindrical number.

Simple Hyperopic Astigmatism

Simple hyeropic astigmatism
Rays of light from one plane meet on the retina but rays of light from another plane 90 degrees away meet behind the retina.  This is known as simple hyperopic astigmatism and is corrected with glasses.  Here too the lenses would only have a cylindrical number.

Compound Myopic Astigmatism

Compund Myopic astigmatism
Rays of light from both planes 90 degrees apart meet in front of the retina.  However, they dont meet at one point.  Thus the patient has myopia and astigmatism.  This is corrected with a minus numbered lens as well as a minus numbered cylinder.  This is known as compound myopic astigmatism.

Compound Hyperopic Astigmatism

compound hyperopic astigmatism
Rays of light from both planes 90 degrees apart meet behind the retina.  However, they dont meet at one point.  Thus the patient has both hyperopia and astigmatism.  This is corrected with a plus numbered lens and a plus numbered cylinder.  This is known as compound hyperopic astigmatism.

Mixed Astigmatism

Compund astigmatism
Rays of light from one plane meet in front of the retina and rays of light from the other plane meet behind the retina.   This is known as Mixed astigmatism and is corrected by a combination of plus and minus lenses.
Pro Tip : When you see your prescription and start analysing your prescription with this information do not worry.  Its not like one is worse than the other. 

How are the axis locations mentioned?

When we prescribe a cylindrical number, we always mention an axis and the lens's power.  

eg : +2.00 DS / -1.50 DC x 180

Here +2.00 is the spherical number.  DS stands for Diopter sphere, and -1.50 is the cylindrical number, where DC is the diopter cylinder.  180 is the axis at which the axis of the cylindrical lens lies.  

Degrees of astigmatism

axes of astigmatism

Is Astigmatism Genetic

We are unsure what causes astigmatism, but genetics is a significant factor. As mentioned, astigmatism is merely not an ideal shape of the cornea. We will often see children diagnosed with astigmatism whose parents also have astigmatism, so genetics plays a significant role.

Types of Astigmatism

Astigmatism types can be broadly divided into two, regular and irregular astigmatism. Since most of us have regular astigmatism, let's discuss that first.

Regular Astigmatism

As the name suggests, regular astigmatism occurs when the principal meridians are 90 degrees apart. There will be a consecutive variation in the refractive power from one meridian to another. Each meridian in the regular astigmatic eye has a uniform curvature at every point across the entrance of the pupil.

There are three types of regular astigmatism

With the rule astigmatism: 

It is known as ' With the rule ' astigmatism when a vertical meridian is steeper than a horizontal meridian. When you have 'with the rule' astigmatism, the axis in your prescription would be between 20 - 160. E.g., -1.00/-1.50*180 

Against the rule astigmatism:

When a horizontal meridian is steeper than a vertical meridian, it is known as 'against the rule' astigmatism. When you have 'against the rule' astigmatism, the axis in your prescription would be between 70-110. E.g., -1.00/1.50*90

Oblique astigmatism:

Oblique astigmatism is when the curvature difference between two meridians is 90 degrees. Still, the location of the steep meridian does not fall within the 'with' or 'against the rule' axes range. If you have oblique astigmatism, the axis in your prescription would be between 20-70 or 110-160. 

Astigmatism can be associated with both Myopia and Hyperopia. Your prescription can be a combination of both, which is fine. Myopia means nearsightedness, and hyperopia means farsightedness. If associated with myopia, it is known as myopic astigmatism and if with hyperopia, as hyperopic astigmatism.

Irregular Astigmatism

Here the principal meridians are not perpendicular or 90 degrees apart. Irregular astigmatism is not as commonly seen as regular astigmatism. Usually, Irregular astigmatism can result from trauma, surgery, or an eye condition known as keratoconus, where the cornea gradually becomes thinner.  

Cause of Astigmatism

Usually, astigmatism is present since birth. However, it can even be developmental. You observe difficulties in children only after the age of 5-7 years before they can get away with it as their world does not require sharp distant vision. In other words, they are concerned only with the near world around them. Unlike Myopia, Astigmatism is usually not of a progressing type. However, if it is progressing continuously, it may be of concern. Progressing astigmatism is usually irregular astigmatism and indicates a corneal disease like keratoconus. Keratoconus is a condition where the shape of the cornea becomes conical, and symptoms can be vision difficulties despite the best possible optical correction. Keratoconus is of significant concern and requires timely intervention because the condition is progressive and can lead to severe visual impairment.  

As age advances, some of us may develop minor astigmatism at the age of 50-60 because of the ageing changes in the lens. These ageing changes are known as Cataracts.  

Thus astigmatism can be corneal or because of the cornea and lenticular or the lens inside the eye. Whenever we give glasses to patients who have astigmatism, we measure the total amount of astigmatism, both corneal and lenticular.

Astigmatism in one eye

Most of the time, the degree of astigmatism is almost equal in both eyes. However, sometimes one may have very different powers in each eye or have a cylindrical number in only one eye. Different eye numbers are picked up in children at a routine eye examination. Many schools require children to submit an eye exam report to detect conditions like these.  

Why?

Because if a child has astigmatism in only one eye, then that eye has blurred vision. The other eye has perfect vision, and thus the child thinks they can see clearly. The child does not complain, and the parents don't bring the child in for an eye exam. Not using one eye in the first eight years of life leads to what is known as amblyopia or a lazy eye.

Test For astigmatism

A routine eye exam is where astigmatism is usually first diagnosed. Parents may complain of their children blurred vision more for far or headaches. Astigmatism is diagnosed while doing refraction or glass power checkup. When we start their regular eye exams, the first thing we find is that they cannot read the entire vision chart or the Snellen chart. Their visual acuity is not normal. We then perform a retinoscopy or autorefractometry, and this eye number can be confirmed by doing subjective refraction. While doing subjective refraction, we use what is known as a Jacksons cross-cylinder of JCC. This lens helps us refine both the cylinder's power and axis.

Sometimes with high astigmatism, your doctor may advise you on the topography of the cornea. Topography is to understand the cornea's shape and rule out an eye disease like keratoconus.

Astigmatism fan test

The astigmatism fan test is a test that helps in identifying the amount and axis of astigmatism each eye may have. While it does give an idea of astigmatism, it is not very accurate and not used these days to measure the amount of cylindrical power.

Astigmatism test online

Online tests are like the astigmatism fan test in one way or another. They do give us an idea of astigmatism but are not accurate enough. In other words, ophthalmologists won't prescribe you glasses based on an online test.

Symptoms of astigmatism

If you have astigmatism, you can have astigmatism symptoms like blurry vision and tilted vision. Individuals who have astigmatism may want to squeeze their eyes to see clearly.

One may even complain of headaches and fatigue by the end of the day. This headache is because if someone has astigmatism and does not wear glasses, they have to strain their eyes to see clearly. Now, straining your eyes is like lifting weights. So if you are lifting even light weights for the whole day, you will finally give in, and the same is happening with your eyes.

Can astigmatism be cured?

Astigmatism treatment is as mentioned above. However, one cannot get rid of astigmatism without any treatment. The cylindrical number cannot decrease, but the good news is most of the time, it does not increase either.

Astigmatism Treatments

Astigmatism is not a disease but merely an imperfection of the front part of the eye. Glasses or contact lenses can correct astigmatism.

Initially, suppose you are a first time wearer of glasses. You may feel slight discomfort and dizziness with the glasses, but don't worry. That is temporary as your eyes are not used to correcting astigmatism. You should feel comfortable after wearing glasses for a week, and you should find improvement in vision with new glasses.

You can also correct astigmatism with contact lenses, known as toric contact lenses, and you should consult with your doctor before trying contact lenses. Your optometrist will train you to wear and remove contact lenses and guide you regarding all dos and don'ts with your contacts.

Pro Tip: Astigmatism correction with toric lenses may not give one crystal clear vision as their glasses can. Most of our patients are happy with toric contact lenses, but a small percentage of them are not.

Contact lenses can be both disposable or soft lenses or rigid contact lenses. Rigid contact lenses are not disposable and are usually used for up to a year. Sometimes when the cylindrical number is high, we prescribe scleral lenses, which are speciality lenses used mainly for irregular corneas.  

Surgery for Astigmatism

Astigmatism Lasik

Yes, Lasik or laser surgery can correct astigmatism too. However, one must keep in mind that this laser surgery cannot correct any amount of astigmatism. Laser eye surgeries can correct up to 5 Diopters. You can read more about refractive surgery.

Astigmatism Cataract surgery 

Cataract surgery has become very advanced. When we perform cataract surgery, we improve vision by removing the cataract and leaving the patient glasses-free for only distance or both distances and near depending on the patient's requirements. When we perform cataract surgery for astigmatism patients, we implant toric IOLs. These toric intraocular lenses can get rid of a significant portion of astigmatism, if not all of it.

Astigmatism ICL

ICL is a surgery done to get rid of eye power. Some of our younger patients, 18-35 years of age, may want to get rid of glasses, but their eye power is very high. In addition to that, if they have thin corneas, they may not be candidates for LASIK. It is in these patients that an ICL surgery is performed. A lens is placed on the eye's natural lens by performing surgery. This artificial lens compensates for the eye power and renders the person glass free. Imagine that you have a contact lens, but I permanently place the contact lens in the eye.  

Clear lens extraction

Clear lens extraction is one of the surgical options for people in the range of 49-55 years of age or around that. Clear lens extraction is also known as refractive lens exchange. This surgery is nothing but cataract surgery. The only difference between this surgery and cataract surgery is the name. The other main difference is the reason for performing surgery. Cataract surgery is performed to eliminate the cataract, and the person also becomes glass free.

What are the effects of astigmatism if left untreated?

If astigmatism is left untreated, it may lead to several concerns; you may have persistent headaches and eye strain, fatigue at the end of the day, with overall tiredness.

Suppose a moderate amount of astigmatism is left untreated. In that case, it can also lead to a condition known as amblyopia (Lazy eye), mainly in children, where an eye with uncorrected refractive error develops poor vision. After the best possible correction, it does not improve as required. Your doctor may advise you of vision therapy and treatment for amblyopia. 

If your astigmatism is progressing and left untreated, there is a significant threat of developing keratoconus, and as discussed earlier, it can lead to non-reversible severe visual impairment.

 

Frequently Asked Questions ?

Can Asttigmatism be cured ?
Doctors cannot cure astigmatism in the true sense.  We cant make it just go away.  However, it can be corrected by either wearing glasses or contact lenses to see clearly.

We can make astigmatism go away by performing LASIK eye surgery.  Here, a laser reshapes the cornea and post the laser, the person can see clearly without glasses or contact lenses.

How is Astigmatism corrected ?
Your doctor can correct astigmatism by wearing glasses or contact lenses.  
One can also get rid of astigmatism by performing Lasik eye surgery.
Another way that someone gets rid of astigmatism is after undergoing cataract surgery or something is known as clear lens extraction.

Which lens is best to correct Astigmatism ?
There are a few types of lenses.
Lenses that go in the glasses
Contact lenses
IOLs or intraocular lenses are used during cataract surgery.  

Cylindrical lenses are used in the glasses to correct astigmatism.  The amount that can be rectified ranges from 0 to 4-5 Diopters of cylindrical number.

The contact lenses are called Toric contact lenses.  There is again an upper limit that can be corrected with contact lenses.  Toric lenses are not available for all types of contact lenses.  E.g. in India, we don't have toric lenses in coloured contact lenses.  For higher powers, one may even use scleral lenses.

IOLs that correct astigmatism are called toric IOLs.  Sometimes, when these lenses are implanted, a small cylinder remains, but yes, these lenses get rid of a significant portion of the cylindrical number.

Can Astigmatism be corrected by LASIK ?
Yes,  it can be corrected by lasik.  There is however a limit.  Usually upto 5 diopters can be corrected by lasik.  If the astigmatism is more than that then one may consider either an ICL surgery or a clear lens extraction.

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